Fuels other than gasoline and diesel are increasingly being used as an energy source for internal combustion engines because of possible reduction in certain regulated emissions. In particular, alcohol and gasoline blends are commonly encountered. The engine air/fuel ratio control system must be able to detect these blends and make adjustments to the engine control parameters in order to maintain optimum operation.
Fuel blending ratio inferring methods are known in which the feedback correction term of the air/fuel ratio control system is monitored. Typical air/fuel ratio control systems use exhaust gas sensors to measure the exhaust air/fuel ratio. If the exhaust sensor indicates the exhaust air/fuel ratio is rich of the desired air/fuel ratio, commonly stoichiometry, then a negative feedback correction term is added to the fuel injected into the engine until the exhaust air/fuel ratio matches the desired air/fuel ratio. Similarly, if the exhaust sensor indicates the exhaust air/fuel ratio is lean of the desired air/fuel ratio, then a positive feedback correction term is added to the fuel injected into the engine until the exhaust air/fuel ratio matches the desired air/fuel ratio. In this way, the measured exhaust air/fuel ratio is made to follow the desired air/fuel ratio.
The fuel blending ratio inferring methods coupled to the above described air/fuel ratio control systems are based on the relationship between the blending ratio and the stoichiometric air/fuel ratio. As the blending ratio changes, and thus the chemical makeup of the fuel changes, the stoichiometric air/fuel ratio changes. When the air/fuel control system uses exhaust gas sensors to maintain stoichiometric combustion, the feedback term will compensate for the change in the stoichiometric air/fuel ratio. By monitoring the size and magnitude of this feedback term, an estimate of the blending ratio can be made. Such a system is disclosed in the U.S. Pat. No. 5,195,497.
The inventors herein have recognized numerous problems with the above approaches. One problem is that errors introduced by the fuel metering system and the air metering system cannot be differentiated from changes in the blending ratio. Stated another way, the feedback term will correct for such errors and also correct for a change in the stoichiometric air/fuel ratio caused by changes in fuel blend. Thus, inferring the blending ratio from the feedback term may result in errors and less than optimal engine operation.